To assess the effect of left ventricular dilatation on mitral valve size, this study compares dimensions of the mitral valve in patients with aortic valve disease and in those with hypertrophic cardiomyopathy (HC). A total of 216 valves, removed at operation or necropsy, was analyzed by quantitative morphometric methods from a) 2 patient groups with dilated left ventricular cavities, i.e., 17 patients with dilated valvular aortic stenosis (AS) and 31 patients with pure aortic regurgitation AR), b) 2 patient groups without dilated left ventricular cavities, i.e., 29 patients with nondilated AS and 94 patients with HC, and c) from 45 control subjects without heart disease. Mean mitral leaflet areas in patients with AS with dilated left ventricular cavities (mean 13.1 plus/minus 3.0 square cm) and AR (mean 12.0 plus/minus 3.6 square cm) were significantly greater than in the controls (8.7 plus/minus 2.0 square cm; p less than 0.001). Mean mitral leaflet area in the patients with AS without dilated left ventricular cavities (9.8 plus/minus 2.0 square cm) were similar to the normal valves (ns). However, mean mitral leaflet area in patients with HC, in whom left ventricular cavities were also nondilated, was significantly greater (12.8 plus/minus 3.7 square cm) compared to that in nondilated AS and in normals (p less than 0.001). Therefore, increased mitral leaflet area: 1) is commonly present in patients with aortic valve disease with dilated left ventricle in whom it appears to be secondary to left ventricular chamber dilatation; and 2) cannot be attributed to left ventricular cavity dilatation in patients with HC, since their cavities were of normal or small size. Thus, increased mitral leaflet area in HC, which is considered to be a primary myocardial and not valvular disease, may be due to a primary effect on the mitral valve.